Rising humidity across India’s coast in summer is a health danger

Before the sun shines on the Arabian Sea, fisherman Mariyani Miyelpillai, 73, is already turning his kattamaram raft homeward. He must escape another summer morning turning too hot and dense to bear.

Fishing solo off the seam where Thiruvananthapuram and Kanyakumari districts meet, his work days have turned into a test of endurance. His raft must be propelled by sheer muscle power, adding to the already challenging heat. “I go for fishing at 5 am, but come back by 8 am, whether I get enough fish or not,” he told Mongabay-India. “I cannot manage this heat.”

As the sun moves past the Tropic of Cancer, ushering in summer, a certain heaviness settles on skin and breath for veteran fishers like Miyelpillai. So does Tarsila Thresya, 54, a fisherwoman. “I go early to sell fish by the roadside. I’m back by 5.30 pm or 6 pm. Earlier it was okay, but now the heat is increasing a lot. Sometimes I put a towel on my head, but it does not help.”

Amidst more frequent and intense heatwaves, the southwestern coast is facing another quieter, insidious shift – rising humid heat. Air so soaked with moisture that sweat no longer cools. On tropical coasts, especially before the monsoon, this is getting closer to dangerous levels as studies show – the human body struggles to cool itself, touching the limits of adaptation.

Closer to home, heat stress along India’s coasts has intensified significantly since 1981, driven by the combined rise in temperature and humidity, a new long-term study led by scientists at the India Meteorological Department shows.

Analysing data from 1981 to 2020, IMD Pune scientist P Rohini and colleagues show that wet-bulb temperatures – a measure of temperature with humidity – have increased across all seasons. The risks are spreading unevenly and largely under-recognised across the country’s humid shorelines.

The trend is clear – a steady shift toward warmer, more moisture-laden air. Extreme heat and humidity events have intensified, particularly since the early 2000s, the IMD study shows. As the climate warms, the atmosphere holds more moisture, amplifying heat stress. This effect is especially pronounced along the east coast, where humidity is rising faster with each degree of warming than on the west coast, as the IMD study shows.

As Rajeevan Madhavan Nair, Vice Chancellor of Atria University, Bengaluru, and former secretary in the Ministry of Earth Sciences, who co-authored and supervised the IMD study, told Mongabay-India, there is a “clear and concerning intensification” of heat stress over recent decades.

He explained that the IMD study shows a significant rise in frequency and duration of heat stress events. It also shows strong warming across both maximum and minimum temperatures, as well as increased thermal discomfort affecting crowded coastal areas. “It has implications for public health, labour productivity, and urban planning,” he noted.

“The findings highlight the urgent need for: heat action plans tailored to coastal microclimates; climate-resilient urban design; early warning systems and adaptive labour policies,” he added.

Scientists measure humid heat using wet-bulb temperature – a metric that combines heat and humidity, unlike the more familiar dry-bulb temperature, which records air temperature alone.

“Housed within a white wooden enclosure, co-located thermometers measure WBT and DBT. The wet-bulb thermometer features a damp wick; as water evaporates, it draws latent heat from the bulb, lowering the reading,” explained P Vijaykumar, assistant professor in the Department of Environmental Science, University of Kerala. “Because the ambient humidity dictates the evaporation rate, higher humidity levels result in less cooling.”

“According to theory, every 1 degrees celsius rise in global temperature results in an approximate 7% increase in atmospheric moisture capacity,” Vijaykumar added. That translates into a double burden of increased humidity and heat.

“While the uneven station distribution and geographical differences between the two coasts limit the robustness of a direct inter-coastal comparison, the IMD study identifies key trends. By integrating reanalysis data – such as sea surface temperature and moisture flux – the study traces decadal shifts in WBT,” he commented.

“The rarity of studies employing four decades – across two periods, 1981-2000 and 2001-2020 – of daily observations underscores the novelty of these findings, which are essential for understanding future coastal heatwave scenarios.”

Scientists note that humid heat has significant health, socio-economic, and environmental implications. “WBT levels are critical determinants of heat exhaustion and heatstroke; these findings have significant implications for human health and safety,” Vijaykumar pointed out.

Extreme heat is not just uncomfortable – it is dangerous. Research shows that heat becomes far more dangerous when combined with high humidity. In such conditions, the human body struggles to regulate its temperature. Normally, sweating helps cool the body – but when the air is already saturated with moisture, this mechanism begins to fail.

It affects human health across a spectrum, from mild heat stress to life-threatening heatstroke. As heatwaves become more frequent, the risks are rising, especially for those with limited access to shelter, healthcare, or cooling, scientists note.

Humidity plays a critical role. As moisture in the air increases, the body’s ability to cool itself through evaporation declines, making even moderate temperatures feel oppressive and, at times, dangerous, even lethal.

Heat stress is a major contributor to weather-related deaths worldwide. It can worsen existing health conditions, including cardiovascular disease and mental health disorders, and place severe strain on vulnerable individuals – particularly the elderly, those with underlying illnesses, and people engaged in outdoor labour.

Physiological studies warn that rising temperatures, coupled with increasing humidity, are pushing human tolerance toward critical limits. As Simon Surinju, 60, a fisherman who launches his kattamaram at 4 am from Vizhinjam harbour in Thiruvananthapuram and heads south toward

Thengapattinam in Kanyakumari on good days, says it tests his limits: “Sometimes it feels like my skin is burning. My body is always sweating. I pour water on myself and paddle fast. Often, my head starts spinning. I feel dizzy.” He now makes a point of returning before 10 am. “I pour water over my head and body. It helps for a while. Then I paddle faster.”

Recent research shows that even the widely cited survivability threshold of 35 degrees celsius wet-bulb temperature may overestimate what the body can endure, especially for older adults and those exposed to the sun or sustained activity. But the impacts are not uniform – age, health, physical fitness, and behaviour all shape how individuals experience and survive heat.

Underneath these trends lies a broader climatic shift. The Indian Ocean is warming, feeding more moisture into the atmosphere and sustaining hot, humid conditions along the coast. As temperatures rise, the air holds more water vapour. The result is not just warmer days, but a different kind of heat altogether — one that lingers, saturates, and presses inward.

Studies now show that extreme humid heat is rising rapidly worldwide, in some places nearing or even exceeding the limits of human survivability. For those who work outdoors — fishers, labourers, vendors — and for the elderly, the risks are immediate and physical.

Lancet Countdown research shows that heat exposure limits labour productivity and adversely affects health. In 2022, India lost an estimated 191 billion potential labour hours due to heat exposure, marking an increase of 54% compared to the 1991-2000 baseline. That means the country potentially lost an estimated $219 billion, equivalent to 6.3% of GDP.

By century’s end, rising heat stress could cut work performance in India by as much as 30%-40%, forcing a rethink of how to protect those who labour under relentless heat, as another study shows.

The IMD study calls for a region-specific heat stress index based on wet-bulb temperature for India. “To achieve this, the operational forecasting system will require the assimilation of high-resolution humidity observations across space and time,” its authors noted. “The forecast must include real-time WBT products and impact-based alerts that convey the hazardous WBT levels into actionable advisories to the public.”

This article was first published on Mongabay.

Source: Scroll
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